1. Field of the Invention
This invention relates to a method and apparatus for controlling a stepping motor in an idling rotational speed control of an electronically controlled motor vehicle engine which computes fuel injection amount, ignition timing, etc. with a microprocessor.
2. Description of the Prior Art
In an idling rotational speed control for a motor vehicle engine, a bypass path is provided parallel to an intake path portion provided with a throttle valve. The sectional area of flow in this bypass path is controlled by a valve body displaced by a stepping motor and, in the idling mode, controlled in relation to the idling rotational speed of an engine. The step value of the stepping motor (that is, the relative degree of opening of the bypass path) is stored in a first memory and a central processing unit operates the stepping motor on the basis of the step value in the first memory. An electronic control comprising the central processing unit and the first memory storage is supplied with a predetermined power in response to opening and closing of an ignition switch in a cab. When the ignition switch is opened, the central processing unit computes the operating amount of the stepping motor on the basis of the difference between a first predetermined value of the step value corresponding to the maximum value of the sectional area of flow in the bypass path and the step value in the first memory storage, and the stepping motor is operated by the amount corresponding to this operating amount according to electric signals from the electronic control so that the step value of the stepping motor becomes the first value. Also, start time control is carried out to make the sectional area of flow in the bypass path equal to a value related to engine temperature when the engine started, to provide a fast idle speed at low temperature. This start time control of the engine is carried out according to initializing signals produced as the line voltage supplied to the electronic control exceeds a predetermined value. This start time control operates the stepping motor on the basis of difference between the first predetermined value and a second predetermined value of the step value corresponding to the sectional area of flow in the bypass path and determined in relation to the engine temperature. However, if the supply of electric power to the electronic control is interrupted, stored values in the first memory storage are erased, and then, as the electric power is returned, the initializing signal is produced so that said start time control is carried out even though it is not actually start time. When start time control is initiated due to such electric power interruptions to the electronic control, problems are encountered causing the operating amount of the stepping motor to be increased, the bypass path to be completely closed and the rotation of the engine to be stopped (engine stop), since the stepping motor is operated on the basis of the difference between the first and second values even though the actual step value of the stepping motor is not the first value.